1. Field of the Invention
The present invention relates to an air intake device of an internal combustion engine.
2. Background Information
In an internal combustion engine, when the intake valve closes, since the air stream flowing within the air intake passage is abruptly stopped by the intake valve, a positive pressure is generated in the intake port located behind the rear face of the intake valve. When such a positive pressure is generated in the intake port, the air located in the intake port flows toward the open end of the air intake passage. As a result, the pressure in the intake port is reduced and a vacuum is produced in the intake port. When a vacuum is produced in the intake port, the air flows toward the intake port and a positive pressure is generated again in the intake port. Consequently, when the engine is operating, a positive pressure and a vacuum are alternately produced in the intake port and a so-called air pulsation is generated. Where such an air pulsation is generated, if the time interval of the opening operation of the intake valve becomes equal to the time period of the air pulsation, a standing wave having a node at the open end of the air intake passage is generated in the air intake passage. When such a standing wave is generated when the intake valve opens, a positive pressure is generated in the intake port and volumetric efficiency is improved. If the time interval of the opening operation of the intake valve becomes equal to the time period of the air pulsation when the engine speed is low, a fundamental standing wave is generated and, if the time interval of the opening operation of the intake valve becomes equal to the time period of the air pulsation when the engine speed is high, a second harmonic standing wave is generated. Consequently, in an engine, a standing wave is generated at a particular engine speed and, at this time, volumetric efficiency is increased.
The engine speed at which such a standing wave is generated depends on the length of the air intake passage. Normally, the generation of the standing wave is discussed by regarding the air intake passage as a straight pipe having a function regarding the generation of the standing wave, which function is the same as that of the air intake passage, and the length of the straight pipe is normally called an equivalent pipe length. An ordinary engine has a fixed equivalent pipe length. Consequently, in such an ordinary engine, the volumetric efficiency is improved at a particular engine speed, but it is impossible to improve the volumetric efficiency over the entire region of the engine speed. However, in such an ordinary engine, if the equivalent pipe length is changed in accordance with a change in the engine speed, it is possible to increase the volumetric efficiency over the entire region of the engine speed, due to the air pulsation.
In order to increase the volumetric efficiency, there is proposed in copending U.S. application No. 06/629,336 filed July 10, 1984, now U.S. Pat. No. 4,565,166. an engine which comprises a main air intake passage connecting the air cleaner to the combustion chambers of the engine, and an auxiliary air intake passage having an equivalent pipe length which is the same as that of the main air intake passage. One of the ends of the auxiliary air intake passage is connected to the air cleaner, and the other end of the auxiliary air intake passage is connected to the main air intake passage. A control valve is arranged in the other end of the auxiliary air intake passage and opens when the engine speed is in a predetermined speed range. In this engine, the equivalent pipe length is changed by opening and closing the control valve and it is possible to increase the volumetric efficiency over the entire region of the engine speed. Where the volumetric efficiency is increased by using the air pulsation, as in the above-mentioned engine, to further increase the volumetric efficiency it is necessary to minimize the attenuation of the air pulsation as much as possible. To this end, it is necessary to minimize the flow resistance of the air intake passage as much as possible. However, in the above-mentioned engine, since the control valve is formed so that it has a large flow resistance, the air pulsation is attenuated by the control valve when the control valve opens. This results in a problem in that a satisfactory high volumetric efficiency cannot be obtained when the control valve opens.